[ALSA] PCM interface - rename SNDRV_PCM_TSTAMP_MMAP to SNDRV_PCM_TSTAMP_ENABLE

Change semantics for SNDRV_PCM_TSTAMP_MMAP. Doing timestamping only in
the interrupt handler might cause that hw_ptr is not related to actual
timestamp. With this change, grab timestamp at every hw_ptr update to
have always valid timestamp + ring buffer position pair.
With this change, SNDRV_PCM_TSTAMP_MMAP was renamed to
SNDRV_PCM_TSTAMP_ENABLE. It's no regression (I think).

Signed-off-by: Jaroslav Kysela <perex@perex.cz>
diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index e6e4aa8..6244911 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -580,9 +580,15 @@
 	if (status->state == SNDRV_PCM_STATE_OPEN)
 		goto _end;
 	status->trigger_tstamp = runtime->trigger_tstamp;
-	if (snd_pcm_running(substream))
+	if (snd_pcm_running(substream)) {
 		snd_pcm_update_hw_ptr(substream);
+		if (runtime->tstamp_mode == SNDRV_PCM_TSTAMP_ENABLE) {
+			status->tstamp = runtime->status->tstamp;
+			goto _tstamp_end;
+		}
+	}
 	snd_pcm_gettime(runtime, &status->tstamp);
+ _tstamp_end:
 	status->appl_ptr = runtime->control->appl_ptr;
 	status->hw_ptr = runtime->status->hw_ptr;
 	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {